/*
 * Copyright (C) 2013 Square, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.squareup.picasso;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;

/**
 * An input stream wrapper that supports unlimited independent cursors for
 * marking and resetting. Each cursor is a token, and it's the caller's
 * responsibility to keep track of these.
 */
final class MarkableInputStream extends InputStream {
	private static final int DEFAULT_BUFFER_SIZE = 4096;

	private final InputStream in;

	private long offset;
	private long reset;
	private long limit;
	private long defaultMark = -1;

	public MarkableInputStream(InputStream in) {
		this(in, DEFAULT_BUFFER_SIZE);
	}

	public MarkableInputStream(InputStream in, int size) {
		if (!in.markSupported()) {
			in = new BufferedInputStream(in, size);
		}
		this.in = in;
	}

	/** Marks this place in the stream so we can reset back to it later. */
	@Override
	public void mark(int readLimit) {
		defaultMark = savePosition(readLimit);
	}

	/**
	 * Returns an opaque token representing the current position in the stream.
	 * Call {@link #reset(long)} to return to this position in the stream later.
	 * It is an error to call {@link #reset(long)} after consuming more than
	 * {@code readLimit} bytes from this stream.
	 */
	public long savePosition(int readLimit) {
		long offsetLimit = offset + readLimit;
		if (limit < offsetLimit) {
			setLimit(offsetLimit);
		}
		return offset;
	}

	/**
	 * Makes sure that the underlying stream can backtrack the full range from
	 * {@code reset} thru {@code limit}. Since we can't call {@code mark()}
	 * without also adjusting the reset-to-position on the underlying stream
	 * this method resets first and then marks the union of the two byte ranges.
	 * On buffered streams this additional cursor motion shouldn't result in any
	 * additional I/O.
	 */
	private void setLimit(long limit) {
		try {
			if (reset < offset && offset <= this.limit) {
				in.reset();
				in.mark((int) (limit - reset));
				skip(reset, offset);
			} else {
				reset = offset;
				in.mark((int) (limit - offset));
			}
			this.limit = limit;
		} catch (IOException e) {
			throw new IllegalStateException("Unable to mark: " + e);
		}
	}

	/** Resets the stream to the most recent {@link #mark mark}. */
	@Override
	public void reset() throws IOException {
		reset(defaultMark);
	}

	/** Resets the stream to the position recorded by {@code token}. */
	public void reset(long token) throws IOException {
		if (offset > limit || token < reset) {
			throw new IOException("Cannot reset");
		}
		in.reset();
		skip(reset, token);
		offset = token;
	}

	/** Skips {@code target - current} bytes and returns. */
	private void skip(long current, long target) throws IOException {
		while (current < target) {
			long skipped = in.skip(target - current);
			if (skipped == 0) {
				if (read() == -1) {
					break; // EOF
				} else {
					skipped = 1;
				}
			}
			current += skipped;
		}
	}

	@Override
	public int read() throws IOException {
		int result = in.read();
		if (result != -1) {
			offset++;
		}
		return result;
	}

	@Override
	public int read(byte[] buffer) throws IOException {
		int count = in.read(buffer);
		if (count != -1) {
			offset += count;
		}
		return count;
	}

	@Override
	public int read(byte[] buffer, int offset, int length) throws IOException {
		int count = in.read(buffer, offset, length);
		if (count != -1) {
			this.offset += count;
		}
		return count;
	}

	@Override
	public long skip(long byteCount) throws IOException {
		long skipped = in.skip(byteCount);
		offset += skipped;
		return skipped;
	}

	@Override
	public int available() throws IOException {
		return in.available();
	}

	@Override
	public void close() throws IOException {
		in.close();
	}

	@Override
	public boolean markSupported() {
		return in.markSupported();
	}
}
